The popularity and convenience of the Internet has resulted in the reinvention of traditional telephony services. These services are offered over a packet switched network with minimal or no cost to the users. IP (Internet Protocol) telephony, thus, have found significant success, particularly in the long distance market. In general, IP telephony, which is also referred to as Voice-over-IP (VOIP), is the conversion of voice information into data packets that are transmitted over an IP network. Such technology has reduced telecommunication costs and enhanced users' convenience in seamlessly employing computing and communication applications and services. The development of telephony services, in this context, poses tremendous engineering challenges in that the merger, for example, of traditional call features, e.g., call waiting, with new capabilities (made possible by the data infrastructure) require preserving interoperability with existing telephony infrastructure. Another driving force for the continual development of these services is the desire for service providers to remain competitive.